The purpose of this study is to evaluate the effects of light on the human circadian clock as measured by circadian rhythms, internal hormones such as melatonin, cortisol, by the circadian rhythm in core body temperature, and by circadian changes in neurobehavioral performance. The results of these studies will have practical significance in the treatment of circadian and sleep disorders and in the development of treatments for shift work and jet-lag dyssomnia.